You did the right thing by contacting the union. I don't think school is obligated to offer you a job share, but your boss is breaking the law by even discussing your date of return! I'm back full-time just before we break up in July, but I was contacted to see if I would come back earlier b/c my cover wasn't working out! i politely said 'no' and then ended the conversation. Forget about you being 'unfair to the children', she's being unfair to you! Surely you're maternity cover was booked to do the entire period? If not, well that's for your head to worry about, not you! Hope you're able to sort things out.